To prepare a polymer of at least 1 μm in mean diameter, a modified radical process such as suspension polymerization, dispersion polymerization, emulsion polymerization or precipitation polymerization is generally used. In suspension polymerization, a monomer and water are stirred at high speed to form droplets, so the resultant polymer has a wide range of mean diameter. Therefore, polymerization has to be performed several times, stage by stage, in order to obtain a target sized polymer. Dispersion polymerization is a method to produce a polymer by using a stabilizer and a solvent in which the monomer is soluble but the produced polymer is insoluble, so the polymerization result can be very vulnerable with the characteristics of a solvent, a stabilizer and their contents. According to emulsion polymerization, the mean diameter of the produced polymer is less than 1 μm. To produce a polymer of at least 1 μm in mean diameter, the swelling method using a seed has to be hired but then reaction process becomes very complicated and reaction time is extended [Colloid Polymer Science Vol. 279, 146-152 (2001)].
In addition to the above radical processes, anion dispersion polymerization can be used to produce a polymer, for example polystyrene [Journal of Polymer Science; Part A; Polymer Chemistry Vol. 34, 2633-2649 (1996)]. This method facilitates a simple and fast reaction and thus favors the production of a monodisperse polymer. However, the monodisperse polymer produced by the anion disperse polymerization does not have such good properties including impact resistance and elastic recovery rate.
With the increasing demand of ultrafine gapping and high connection density in circuit of electronic packaging such as LCD, PDP, OLED, etc, it is required to connect numbers of electrodes at a time. In particular, conductive adhesive for mechanical and electric connection of FPC (Flexible Printed Circuit) to glass display has been used in LCD packaging.
The conductive adhesive is largely divided into isotropic conductive adhesive and anisotropic conductive film (ACF), and basically they all have the same structure in which monodisperse conductive particles (conductive balls) are dispersed in a thermosetting or thermoplastic insulating resin.
The conductive ball is composed of an insulating, cross-linked monodisperse polymer core particle of 1-10 μm in diameter having preferable elastic modulus and recovery rate and the surface of the polymer particle is coated with a metal such as Ni or Au to increase conductivity. The monodisperse polymer particle can be formed by polystyrene resin, polyacrylate resin and urethane resin, etc.
The monodisperse polystyrene resin prepared by anion dispersion polymerization can be used for the conductive ball, but in that case the improvement of physical properties including impact resistance and elastic recovery rate is required.